arXiv subjects

Zhixiang Shi

Publications and source records attributed to Zhixiang Shi.

At least 55 records · Page 3Linked to original sources

arXiv2021

Hydrothermal synthesis and complete phase diagram of FeSe$_{1-x}$S$_{x}$ $(0 \leq x \leq 1)$ single crystals

We report the successful synthesis of FeSe$_{1-x}$S$_{x}$ single crystals with $x$ ranging from 0 to 1 via a hydrothermal method. A complete phase diagram of FeSe$_{1-x}$S$_{x}$ has been obtained based on resistivity and magnetization measurements. The nematicity is suppressed with increasing $x$, and a small superconducting dome appears within the nematic phase. Outside the nematic phase, the superconductivity is continuously suppressed and reaches a minimum $T_c$ at $x$ = 0.45; beyond this point, $T_c$ slowly increases until $x$ = 1. Intriguingly, an anomalous resistivity upturn with a characteristic temperature $T^*$ in the intermediate region of $0.31 \leq x \leq 0.71$ is observed. $T^{*}$ shows a dome-like behavior with a maximum value at $x$ = 0.45, which is opposite the evolution of $T_c$, indicating competition between $T^*$ and superconductivity. The origin of $T^*$ is discussed in detail. Furthermore, the normal state resistivity evolves from non-Fermi-liquid to Fermi-liquid behavior with S doping at low temperatures, accompanied by a reduction in electronic correlations. Our study addresses the lack of single crystals in the high-S doping region and provides a complete phase diagram, which will promote the study of relations among nematicity, superconductivity, and magnetism.

cond-mat.supr-con↗

arXiv2020

Fully gapped superconductivity without sign reversal in the topological superconductor PbTaSe$_2$

We investigate the superconducting gap function of topological superconductor PbTaSe$_2$. Temperature, magnetic field, and three-dimensional (3D) field-angle dependences of the specific heat prove that the superconductivity of PbTaSe$_2$ is fully-gapped, with two isotropic $s$-wave gaps. The pair-breaking effect is probed by systematically increasing non-magnetic disorders through H$^+$-irradiations. The superconducting transition temperature, $T_{\rm{c}}$, is found to be robust against disorders, which suggests that the pairing should be sign-preserved rather than sign-reversed.

cond-mat.supr-con↗

arXiv2020

Angle-dependent magnetoresistance and its implications for Lifshitz transition in W2As3

Lifshitz transition represents a sudden reconstruction of Fermi surface structure, giving rise to anomalies in electronic properties of materials. Such a transition does not necessarily rely on symmetry-breaking and thus is topological. It holds a key to understand the origin of many exotic quantum phenomena, for example the mechanism of extremely large magnetoresistance (MR) in topological Dirac/Weyl semimetals. Here, we report studies of the angle-dependent MR (ADMR) and the thermoelectric effect in W2As3 single crystal. The compound shows a large unsaturated MR (of about 70000% at 4.2 K and 53 T). The most striking finding is that the ADMR significantly deforms from the horizontal dumbbell-like shape above 40 K to the vertical lotus-like pattern below 30 K. The window of 30-40 K also corresponds substantial changes in Hall effect, thermopower and Nernst coefficient, implying an abrupt change of Fermi surface topology. Such a temperature-induced Lifshitz transition results in a compensation of electron-hole transport and the large MR as well. We thus suggest that the similar method can be applicable in detecting a Fermi-surface change of a variety of quantum states when a direct Fermi-surface measurement is not possible.

cond-mat.mtrl-sci↗

arXiv2020

Accurate Temporal Action Proposal Generation with Relation-Aware Pyramid Network

Accurate temporal action proposals play an important role in detecting actions from untrimmed videos. The existing approaches have difficulties in capturing global contextual information and simultaneously localizing actions with different durations. To this end, we propose a Relation-aware pyramid Network (RapNet) to generate highly accurate temporal action proposals. In RapNet, a novel relation-aware module is introduced to exploit bi-directional long-range relations between local features for context distilling. This embedded module enhances the RapNet in terms of its multi-granularity temporal proposal generation ability, given predefined anchor boxes. We further introduce a two-stage adjustment scheme to refine the proposal boundaries and measure their confidence in containing an action with snippet-level actionness. Extensive experiments on the challenging ActivityNet and THUMOS14 benchmarks demonstrate our RapNet generates superior accurate proposals over the existing state-of-the-art methods.

cs.CV↗

arXiv2019

Review of annealing effects and superconductivity in Fe$_{1+y}$Te$_{1-x}$Se$_x$ superconductors

Fe$_{1+y}$Te$_{1-x}$Se$_x$ is unique in their structural simplicity, consisting of only FeTe/Se layers, which is favorable for probing the mechanism of superconductivity. Recently, a topological surface superconductivity as well as the Majorana Fermions has been observed, which makes Fe$_{1+y}$Te$_{1-x}$Se$_x$ the first high temperature topological superconductor. Since large size single crystal of Fe$_{1+y}$Te$_{1-x}$Se$_x$ can be easily grown, many researches have been performed. However, a large part of the reported results are under controversy, including the resistivity, susceptibility, Hall effect, gap structure, phase diagram, etc. These controversies are believed to come from the sample-dependent Fe nonstoichiometries, which originate from the partial occupation of the second Fe site (excess Fe) in the Te/Se layer. The excess Fe with valence near Fe$^+$ will provide electron doping into the system. Meanwhile, the excess Fe is also strongly magnetic, which will act as a paring breaker and also localize the charge carriers. Removing the excess Fe is essential to probe the intrinsic properties and mechanism of superconductivity of Fe$_{1+y}$Te$_{1-x}$Se$_x$ compounds. In this topical review, we propose the effective approaches to remove excess Fe in Fe$_{1+y}$Te$_{1-x}$Se$_x$. Furthermore, we discuss the mechanism of annealing based on the evolutions of structure, composition, and morphology with annealing. Moreover, we also review the annealing effects on the normal state and superconducting properties, including the magnetism, transport properties, band structure, $T_{\rm{c}}$, phase diagram, upper critical field, anisotropy, critical current density, gap structure, and superconducting pairing. This review presents not only the optimal way to prepare crystals without excess Fe, but also the intrinsic properties of Fe$_{1+y}$Te$_{1-x}$Se$_x$ without the influence of excess Fe.

cond-mat.supr-con↗

arXiv2019

Relation-Aware Pyramid Network (RapNet) for temporal action proposal

In this technical report, we describe our solution to temporal action proposal (task 1) in ActivityNet Challenge 2019. First, we fine-tune a ResNet-50-C3D CNN on ActivityNet v1.3 based on Kinetics pretrained model to extract snippet-level video representations and then we design a Relation-Aware Pyramid Network (RapNet) to generate temporal multiscale proposals with confidence score. After that, we employ a two-stage snippet-level boundary adjustment scheme to re-rank the order of generated proposals. Ensemble methods are also been used to improve the performance of our solution, which helps us achieve 2nd place.

cs.CV↗

arXiv2019

Quasi-particle evidence for the nematic state above $T_{\rm{c}}$ in Sr$_x$Bi$_2$Se$_3$

In the electronic nematic state, an electronic system has a lower symmetry than the crystal structure of the same system. Electronic nematic states have been observed in various unconventional superconductors such as cuprate- and iron-based, heavy-fermion, and topological superconductors. The relation between nematicity and superconductivity is a major unsolved problem in condensed matter physics. By angle-resolved specific heat measurements, we report bulk quasi-particle evidence of nematicity in the topological superconductor Sr$_x$Bi$_2$Se$_3$. The specific heat exhibited a clear 2-fold symmetry despite the 6-fold symmetric lattice. Most importantly, the 2-fold symmetry appeared in the normal state above the superconducting transition temperature. This is explained by the angle-dependent Zeeman effect due to the anisotropic density of states in the nematic phase. Such results highlight the interrelation between nematicity and unconventional superconductivity.

cond-mat.supr-con↗

arXiv2019

Deviation from canonical collective creep behavior in Li$_{0.8}$Fe$_{0.2}$OHFeSe

We present an extensive study of the vortex dynamics in Li$_{0.8}$Fe$_{0.2}$OHFeSe single crystal, which is prepared by intercalating insulating spacer layer Li$_{0.8}$Fe$_{0.2}$OH into FeSe. Absence of fishtail effect and the crossover from elastic to plastic creep are observed similar to the parent compound FeSe. However, when we apply collective creep theory to the magnetic relaxation data, the obtained creep exponent $μ$ is found to be $\sim$ 4.1 much larger than the predicted maximum value. Besides, the elastic creep in the vortex phase diagram of Li$_{0.8}$Fe$_{0.2}$OHFeSe is found to reside only in an extremely small region. Such a large value of $μ$ and the small elastic creep region may be originated from the weakening of coupling between the vortices in neighbouring layers by elongating the layer distance. It may indicate that the vortex structure of Li$_{0.8}$Fe$_{0.2}$OHFeSe is in the crossover regime between elastic Abrikosov vortices to stacks of pancake vortices. Our study also suggests that the intercalated FeSe is a promising candidate for searching intrinsic Josephson phenomena.

cond-mat.supr-con↗

arXiv2018

Universal scaling behavior of the upper critical field in strained FeSe0.7Te0.3 thin films

Revealing the universal behaviors of iron-based superconductors (FBS) is important to elucidate the microscopic theory of superconductivity. In this work, we investigate the effect of in-plane strain on the slope of the upper critical field Hc2 at the superconducting transition temperature Tc (i.e. -dHc2/dT) for FeSe0.7Te0.3 thin films. The in-plane strain tunes Tc in a broad range, while the composition and disorder are almost unchanged. We show that -dHc2/dT scales linearly with Tc, indicating that FeSe0.7Te0.3 follows the same universal behavior as observed for pnictide FBS. The observed behavior is consistent with a multiband superconductivity paired by interband interaction such as sign change s+- superconductivity.

cond-mat.supr-con↗

arXiv2017

The influence of the in-plane lattice constant on the superconducting transition temperature of FeSe0.7Te0.3 thin films

Epitaxial Fe(Se,Te) thin films were prepared by pulsed laser deposition on (La0.18Sr0.82)(Al0.59Ta0.41)O3 (LSAT), CaF2-buffered LSAT and bare CaF2 substrates, which exhibit an almost identical in-plane lattice parameter. The composition of all Fe(Se,Te) films were determined to be FeSe0.7Te0.3 by energy dispersive X-ray spectroscopy, irrespective of the substrate. Albeit the lattice parameters of all templates have comparable values, the in-plane lattice parameter of the FeSe0.7Te0.3 films varies significantly. We found that the superconducting transition temperature (Tc) of FeSe0.7Te0.3 thin films is strongly correlated with their a-axis lattice parameter. The highest Tc of over 19 K was observed for the film on bare CaF2 substrate, which is related to unexpectedly large in-plane compressive strain originating mostly from the thermal expansion mismatch between the FeSe0.7Te0.3 film and the substrate.

cond-mat.supr-con↗

arXiv2016

Anisotropic Ginzburg-Landau scaling of Hc2 and transport properties of 112-type Ca0.8La0.2Fe0.98Co0.02As2 single crystal

High-quality single crystal of Ca0.8La0.2Fe0.98Co0.02As2 has been successfully synthesized using a self-flux method. The magnetization measurement reveals a second peak effect and high critical current density exceeding 2*10^6 A/cm2 at 5 K(self-field). The upper critical field anisotropy was systematically studied by measuring the electrical resistivity under various magnetic fields and angles. The angle dependent magnetoresistance, by choosing an appropriate anisotropy parameter within the framework of the anisotropic Ginzburg Landau (AGL) theory, can be scaled onto one single curve. In the normal state, the negative Hall coefficient shows strong but nonmonotonic T-dependence through a minimum at 175 K. Moreover, it is shown that the magnetoresistance apparently violates the semiclassical Kohler's rule below 175 K but can be well scaled by the Hall angle instead. This suggests either the change of carriers with T or the exotic anisotropic scattering in the system.

cond-mat.supr-con↗

arXiv2015

Influence of substrate type on transport properties of superconducting FeSe0.5Te0.5 thin films

FeSe0.5Te0.5 thin films were grown by pulsed laser deposition on CaF2, LaAlO3 and MgO substrates and structurally and electro-magnetically characterized in order to study the influence of the substrate on their transport properties. The in-plane lattice mismatch between FeSe0.5Te0.5 bulk and the substrates shows no influence on the lattice parameters of the films, whereas the type of substrates affects the crystalline quality of the films and, therefore, the superconducting properties. The film on MgO showed an extra peak in the angular dependence of critical current density Jc(θ) at θ = 180° (H || c), which arises from c-axis defects as confirmed by transmission electron microscopy. In contrast, no Jc(θ) peaks for H || c were observed in films on CaF2 and LaAlO3. Jc(θ) can be scaled successfully for both films without c-axis correlated defects by the anisotropic Ginzburg-Landau (AGL) approach with appropriate anisotropy ratio γJ. The scaling parameter γJ is decreasing with decreasing temperature, which is different from what we observed in FeSe0.5Te0.5 films on Fe-buffered MgO substrates.

cond-mat.supr-con↗

arXiv2014

Unabridged phase diagram for single-phased FeSexTe1-x thin films

A complete phase diagram and its corresponding physical properties are essential prerequisites to understand the underlying mechanism of iron based superconductivity. For the structurally simplest 11 (FeSeTe) system, earlier attempts using bulk samples have not been able to do so due to the fabrication difficulties. Here, thin FeSexTe1-x films with the Se content covering the full range were fabricated by using pulsed laser deposition method. Crystal structure analysis shows that all films retain the tetragonal structure in room temperature. Significantly, the highest superconducting transition temperature (TC = 20 K) occurs in the newly discovered domain, 0.6 - 0.8. The single-phased superconducting dome for the full Se doping range is the first of its kind in iron chalcogenide superconductors. Our results present a new avenue to explore novel physics as well as to optimize superconductors.

cond-mat.supr-con↗

arXiv2014

Bulk Superconductivity in Fe1+yTe0.6Se0.4 Induced by Removal of Excess Fe

Experimental evidences from transport, magnetic, and magneto-optical (MO) image measurements confirmed that arsenic (As) vapor annealing was another effective way to induce bulk superconductivity with isotropic, large, and homogenous superconducting critical current density (Jc) in Fe1+yTe0.6Se0.4 single crystal. Since As is an exotic and easily detectable heavy element to Fe1+yTe0.6Se0.4 single crystal, As vapor annealing is very advantageous for the study of annealing mechanism. Detailed micro-structural and elemental analyses exclude the possibility that intercalating or doping effect may happen in the other post-annealing methods, proving that Fe reacts with As on the surface of the crystal and the reaction itself acts as a driving force to drag excess Fe out. The removal of excess Fe results in the good superconductivity performance.

cond-mat.supr-con↗

arXiv2014

Anisotropic Superconductivity of Ca1-xLaxFeAs2 (x ~ 0.18) Single Crystal

Anisotropic superconducting properties including the upper critical field Hc2, thermal activation energy U0, and critical current density Jc are systematically studied in a large Ca1-xLaxFeAs2 single crystal (x ~ 0.18). The obtained Hc2 bears a moderate anisotropy gamma of approximately 2-4.2, located between those of '122' Ba1-xKxFe2As2 (1 < gamma < 2) and '1111' NdFeAsO1-xFx (5 < gamma < 9.2). Both the magnitude of U0 and its field dependence are very similar to those of NdFeAsO1-xFx, also indicating anisotropic superconductivity. Moreover, high and anisotropic Jc's exceeding 10^5 A/cm2 have been calculated from the magnetization hysteresis loops, indicating the existence of strong bulk-dominated pinning in the present superconducting material.

cond-mat.supr-con↗

arXiv2014

Multiband Effect and Possible Dirac Fermions in Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$

We investigated the transport properties of Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ single crystals with different amounts of excess Fe prepared by O$_2$ annealing. The O$_2$ annealing remarkably improves transport properties. In particular, a strongly nonlinear Hall resistivity was observed only in the fully-annealed crystal, and the magnetoresistance (MR) is drastically enhanced after annealing, reaching a value larger than 17% at 16 K and 14 T. The obvious change of transport properties after the annealing indicates that the band structure of Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ is affected by the excess Fe. The nonlinear Hall resistivity and violation of (modified) Kohler's scaling of the large MR prove the multiband effects in the Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ single crystal. The MR for the fully-annealed crystal develops linearly against magnetic field from intermediate field (e. g. 2 T at 16 K) to the measurement limit of 14 T. This phenomenon is interpreted by the existence of Dirac cone state, in which all the Dirac fermions occupy only the lowest Landau level in the quantum limit.

cond-mat.supr-con↗

arXiv2014

Dynamics and mechanism of oxygen annealing in Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ single crystal

Iron chalcogenide Fe(Te,Se) attracted much attention due to its simple structure, which is favorable for probing the superconducting mechanism. Its less toxic nature compared with iron arsenides is also advantageous for applications of iron-based superconductors. By intercalating spacer layers, superconducting transition temperature has been raised over 40 K. On the other hand, the presence of excess Fe is almost unavoidable in Fe(Te,Se) single crystals, which hinders the appearance of bulk superconductivity and causes strong controversies over its fundamental properties. Here we report a systematical study of O$_2$-annealing dynamics in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ by controlling the amount of O$_2$, annealing temperature, and time. Bulk superconductivity can be gradually induced by increasing the amount of O$_2$ and annealing time at suitable temperatures. The optimally annealed crystals can be easily obtained by annealing with ~ 1.5% molar ratio of oxygen at 400 $^{\circ}$C for more than 1 hour. Superconductivity was witnessed to evolve mainly from the edge of the crystal to the central part. After the optimal annealing, the complete removal of excess Fe was demonstrated via STM measurements. Some fundamental properties were recharacterized and compared with those of as-grown crystals to discuss the influence of excess Fe.

cond-mat.supr-con↗

arXiv2013

Bulk Superconductivity in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ Induced by Annealing in Se and S Vapor

We reported that bulk superconductivity in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ can be induced by annealing in Se and S vapor. The well-annealed samples show a large critical current density, emph{J}$_c$ $\sim$ 2 - 4 $\times$ 10$^5$ A/cm$^2$. Combined with our previous reports about O$_2$ and Te annealing effects, we proved that bulk superconductivity can be successfully induced in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ by annealing in atmosphere of all chalcogens, such as O, S, Se and Te.

cond-mat.supr-con↗